Preheat the oven to 350 degrees F (175 degrees C). Grease an 8-inch square baking dish.
Mix peaches, bananas, 2 tablespoons flour, and 1 teaspoon sugar substitute in a bowl until fruit is coated; spread into the bottom of the prepared baking dish.
Whisk 1 cup flour, baking powder, salt, and cinnamon in the same bowl used to mix fruit. Stir in skim milk, egg, corn oil, remaining 2 teaspoons sugar substitute, and vanilla until well combined. Pour batter in an even layer over fruit.
Bake in the preheated oven until cobbler is golden brown and a toothpick inserted into the middle comes out clean, about 30 minutes. Remove to a wire rack and cool in the pan for at least 20 minutes before serving.